Presentation

Polymyositis tends to become evident in
adulthood, presenting with bilateral proximal muscle weakness,
often noted in the upper legs due to early fatigue
while walking. Sometimes the weakness presents itself by the person
being unable to rise from a seated position without help, or
inability to raise their arms above their head. The weakness is
generally progressive, accompanied by lymphocytic inflammation
(mainly cytotoxic T8 lymphocytes).

Polymyositis, like dermatomyositis, strikes
females with greater frequency than males. The skin involvement of
dermatomyositis is absent in polymyositis.

Sporadic inclusion
body myositis (sIBM): IBM is often confused with (misdiagnosed
as) polymyositis and polymyositis that does not respond to
treatment is likely IBM. sIBM comes on over months to years,
polymyositis comes on over weeks to months. It appears that sIBM
and polymyositis share some common features, especially the initial
sequence of immune system activation, however, polymyositis does
not display the subsequent muscle degeneration and protein
abnormalities as seen in IBM. As well, polymyositis tends to
respond well to treatments, IBM does not. IBM and polymyositis
apparently involve different disease mechanisms than are seen in
dermatomyositis.